Customarily, cathode ray tubes utilized as color television picture tubes employ a color screen in combination with a compatible grid or other apertured structure such as the shadow mask type construction. The screen, which is normally disposed on the inner surface of the face panel of the tube, is composed of a large number of dot or slot-like formations of electron-responsive green, blue, and red cathodoluminescent phosphor materials arranged in a specifically-defined pattern. The respective cathodoluminescent groupings (frequently termed color triads) constituting the patterned screen are formed in accordance with the number of electron guns to be utilized in the subject tube and with the respective apertured shadow mask employed.
To provide a picture of high resolution, a very large number of the color triads is required which must be set in the proper pattern. Therefore, the process for forming the pattern of color triads must be one which is capable of accurately forming discrete configurations. One example of a conventional process involves a photoprinting technique wherein the inner surface of the face panel, which has a coating of a light sensitive substance with a desired cathodoluminescent phosphor material contained therewithin, is exposed to a specific controlled directional source of light, the beams of which pass through the apertures in the adjacently-positioned mask to impinge upon the screen coating therebeneath. Subsequent etching of the screen produces the first cathodoluminescent pattern consisting of a spaced array of phosphor dots or slots capable of a particular color cathodoluminescence. That process is sequentially repeated with the other color cathodoluminescent phosphor materials to complete the formation of the tri-color screen. Thus, three separate coatings and exposures thereof are required. In the exposure of each color portion of the screen, the directional source of light is appropriately offset during the exposure operation to provide individual color emitting cathodoluminescent material patterns which are discretely spaced apart from one another such as to form a repetitive plurality of color triads constituting the screen.
Conventionally, the shadow mask is of a foraminous nature such that the resultant screen on the inner surface of the viewing panel is comprised of oriented triads of green, blue, and red phosphor dots or slots. Since the color spots are photodisposed by light beamed through the foraminous mask, the spots are desirably in registry or alignment along the electron beam paths with the apertures of the shadow mask.
In a color picture tube there is a trade-off between picture contrast and brightness. Contrast is reduced as the ambient light level is increased. For example, some television sets produce a picture which is scarcely visible in the presence of sunlight. The reason underlying that phenomenon is the fact that white light is reflected from the phosphor spots, thereby mixing unwanted colors with the light emitted from the phosphor, as well as reducing the light-to-dark ratio (contrast). In an effort to decrease that effect, a neutral gray tint is customarily incorporated in the panel glass.
However, it has been recognized that a much better solution to the problem would be to apply a spectrally-selective filter to the face panel in front of the phosphor, e.g., a blue filter in front of the blue phosphor and a red filter in front of the red phosphor. Hence, if the above-described three-phosphor triads were applied in registry with a corresponding three-color filter array, the perceived amount of light reflected from the red and blue phosphor spots would be reduced significantly.
A second utility that can result from the development of color-triad arrays in the face plate of a color television tube is the capability of employing a single "white" phosphor coating and using the color-triad arrays to produce the desired color information. Such a process would eliminate the costly and complex conventional three-step method described above for applying the three-phosphor triad.